Process for anodic oxidation of aluminum and its alloys



Patented Feb. 5, 1 952 UNITED STATES PATENT- OFFICE PROCESS FOR ANonio OXIDATION F ALUMINUM AND ITS ALLOYS Carlo Sonnino, Milan, Italy No Drawing. Application May 29, 1947, Serial No. 751,453. In Switzerland October 20, 1944 Section 1, Public Law 690, August 8, 1946. Patent expires ()ctober 20, 1964 4 Claims. (Cl. 204-58) time, if beryllium sulphate and/or beryllium salts are added to an electrolyte composed of water and sulphuric acid. The surprising feature'is that layers obtained according to the invention can be colored, notwithstanding their extraordinary hardness.

The layer is colourless and transparent, vitreous and very hard, and in spite of that it can be colored by dipping or by any suitable system, either by the direct application of colors or by chemical reaction.

The time of oxidation may be from 20 minutes to an hour and the thickness of the layer obtained is 0.06 mm. The hardness is extraordinary and scleroscope hardnesses for different alloys, expressed in grams of load required for penetration of diamond tip through the whole depth of oxyde layer, are:

Layer according Maximum of to the invention other methods Polyvalent alcohols can be advantageously added to the electrolytes containing beryllium sulphate or other beryllium salts according to the invention.

Two difierent types of layers may be obtained by varying the percentage of sulphuric acid and beryllium sulphate in the bath, namely:

1. A very hard and scarcely flexible layer suitable to prevent undesirable locking of connections, bolts or threaded elements of aluminium or its alloys to be screwed or forced in other threaded pieces of aluminium or its alloys, said layer being considerably porous so that it is possible to color, by absorption from coloring solutions, pieces intended to undergo frictional wear.

2. A less hard but very porous and flexible layer, such as will allow press-molding of oxidized articles or the formation of a colored or white pigment in said layer so as to give a lacquer appearance to the objects, maintaining, however, the features of the anodic film.

Hereinafter I will describe two examples for carrying out the method according to the invention corresponding to said two types of layer:

Example 1 (1st type layer) 1 kg. of beryllium sulphate is added, after cooling, to a bath composed of 15 kg. of sulphuric acid at 66 B. and 85.1 of distilled water.

The optimum operating range with this bath is from 13 to 21 C., with 9-15 volts (preferably 12.5 volts) and about 0.8 ampere/dm.'-, aluminum semi-hard plate, rolled Duralumin.

Example 2 (2nd type layer) 1.5-2.5 kg. of beryllium sulphate are added. after cooling, to a bath composed of 30 kg. of sulphuric acid at 66 B. and 70 l. of distilled water.

The operating range is of 12 volts and the current density of 0.8 ampere/dm. for rolled or drawn aluminum or aluminum alloys, or of 0.5 ampere/dm. for cast aluminum alloys, the temperature being between 18 and 23 0., according to whether color or pigment is to be absorbed.

Tested hardness values are somewhat smaller than in the first example and almost the same for different alloys, but still greater than those obtained by other methods: the value for aluminum, for instance, is 100 g. compared with 80 g. obtained by other methods.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:

1. In a process for the production of transparent and hard coatings on aluminum and its alloys the step which comprises making said metal the anode in an electrolytic bath consisting essentially of about 84 parts by weight of water, 15 parts of sulphuric acid and 1 part of beryllium sulphate.

2. Process as claimed in claim 1 in which said bath is maintained at a temperature of 13 to 21 C. and in which the electric current passed through the bath has an intensity of about 0.8

- ampere per Sq. dm.

3. In a process for the production of transparent and hard coatings on aluminum and its 4. Process as claimed in claim 1 in which said bath is maintained at a temperature from 18 to 23 C., and in which the electric current passed through the bath has a potential of 12 volts and an intensity oi Ufa-V0.8 ampere per sq. dm..

' CARLO SONNINO.

REFERENCES CITED UNITED STATES PATENTS Name Date Tosterud Oct. 22, 1935 Bengston Dec. 3, 1935 Schenk Oct. 21, 1941 Beebe June 8, 1943 FOREIGN PATENTS Country Date Germany Apr. 30, 1941 

1. IN A PROCESS FOR THE PRODUCTION OF TRANSPARENT AND HARD COATINGS ON ALUMINUM AND ITS ALLOYS THE STEP WHICH COMPRISES MAKING SAID METAL THE ANODE IN AN ELECTROLYTIC BATH CONSISTING ESSENTIALLY OF ABOUT 84 PARTS BY WEIGHT OF WATER, 15 PARTS OF SULPHURIC ACID AND 1 PART OF BERYLLIUM SULPHATE. 